Abstract:Welding is a widely adopted industrial process used for joining components. A fusion weld has a highly heterogeneous microstructure and characterisation of strength heterogeneity is difficult because of the potentially large variations over a limited distance. Hardness mapping and miniature tensile tests are two distinct approaches to this problem. This paper reports on the possibilities and limitations of both techniques. Hardness mapping is a well-documented procedure as opposed to miniature tensile testing, where the dimensions of the dogbone shaped specimens are smaller than what standards prescribe. A particular challenge is the measurement of strains in such small specimens. The authors have achieved this measurement by means of Digital Image Correlation (DIC). To that end, a sufficiently fine speckling method has been developed.Keywords: heterogeneity; hardness mapping; miniature tensile testing; DIC INTRODUCTIONWelding is one of the most common joining methods due to its generally high production speed and low associated costs. It is often associated with flexibility, integrity and reliability [1]. An example application showing the economic importance of welding is pipeline installation. Costs associated to fusion welding are estimated to make up 20% of the total cost of pipe laying. These include aligning the pipe sections, cleaning and grinding, welding, checking the weld for flaws and coating the weld [2].Notwithstanding the economical appeal of welding, the localized application of heat and melting results in a high susceptibility to material discontinuities, flaws and residual stresses, whose presence may lead to structural failure and lifetime reduction. The prediction of weld flaw acceptability ('structural integrity') is hampered by the potential presence of strongly heterogeneous microstructures. Modern day weld flaw assessment ignores this heterogeneity by assuming a homogeneous weld. As such, a generally accepted method to characterise weld heterogeneity does not yet exist. STATE OF THE ARTHeterogeneity is inherent to fusion welds, because in order to melt the weld metal and fuse it with the base metal, heat has to be locally applied. For thicker sections, weldments are created in several passes (deposition of multiple layers), resulting in a variety of zones undergoing a succession of particular heat cycles. The heat cycle determines what the microstructure transforms into, each structure having a different stress-strain response. Not only the weld metal is heterogeneous due to this, but the heat affected zone (HAZ) of the base metal (heated but non-melted metal in close vicinity of the weld) also transforms.Many conventional steel welds show a hardened HAZ due to the occurrence of stronger microstructures. In contrast, a broad range of high strength low alloy steels obtain their properties from a fine grain size, resulting from a specific heat treatment, often combined with mechanical deformation. These steels may show a softened HAZ due to grain coarsening. To quantify variations ...

The heterogeneity of arc-welded connections is often ignored in structural assessments, giving rise to inaccuracies. Improved assessments taking into account heterogeneity require the characterization of local constitutive properties. We have compared two methods to do this: Vickers-hardness mapping and miniature tensile testing. Whereas the former is more straightforward to apply, the latter provides full-range stress-strain data. This paper discusses an experimental comparison of both methods on a heterogeneous arc weld. Miniature tensile tests were performed, using digital image correlation to measure the strain. The specimens were indented to compare their stress-strain response with Vickers hardness. Notwithstanding that small natural flaws invalidated some tests, reliable stress-strain curves were obtained. Vickers hardness testing is a convenient alternative if the yield and ultimate tensile strength are the only points of interest and the corresponding conversion inaccuracy is acceptable. Keywords: arc weld, heterogeneity, hardness, miniature tensile testing, digital image correlation Heterogenost oblo~no zavarjenega zvara pogosto ni upo{tevana pri ocenjevanju celovitosti konstrukcij, kar je lahko razlog za nepravilno oceno. Za bolj{o oceno, ki upo{teva heterogenost, je potrebno dolo~iti lokalne mehanske lastnosti. Avtorji~lanka so opravili primerjavo dveh metod za dolo~anje lokalnih mehanskih lastnosti: matri~no meritev mikrotrdote po Vickersu in miniaturni natezni preizkus. Za razliko od dosedanje prakse neposredne uporabe mejnih vrednosti, se danes uporablja celotna natezna krivulja napetost-deformacija. V~lanku je opravljena primerjava med obema eksperimentalno dobljenima skupinama podatkov za heterogeni zvarni spoj. Meritve deformacij na miniaturnih nateznih preizku{ancih so bile opravljene s stereo opti~no digitalno merilno metodo. Na vsakemu od miniaturnih nateznih preizku{ancev je bila opravljena meritev mikrotrdote po Vickersu, pri~emer so bile izmerjene vrednost mikrotrdote primerjane z natezno krivuljo napetost-deformacija. Kljub dejstvu, da so bili preizku{anci z majhnimi napakami v zvarih izklju~eni iz analize, so bile dobljene verodostojne vrednosti meje te~enja in natezne trdnosti iz krivulje napetost-deformacija. Pokazalo se je, da je za hitro oceno natezne trdnosti in meje te~enja materiala zvara, matri~na meritev mikrotrdote po Vickersu alternativna metoda nateznemu preizku{anju. Klju~ne besede: oblo~ni zvar, heterogenost, trdota, miniaturno natezno preizku{anje, digitalne opti~ne meritve